Few things I forgot to say (It won't let me edit the first message)
I am thinking that socket 1366 is the best option, since it has an upgrade path to the hex core intel CPU's coming in the future. But long term, I am not sure if 1366 is going to be a good option, maybe 1156 and i7 8xx are the best choice right now.
Also, I listed stock cooler for overclock, but I am not entirely against using a 3rd party heatsink, but it seems like a more complicated process, with thermal grease etc, so I am not particularly confident.
That's all I can think of right now. But no doubt something else I forgot to mention will pop into my head after I post this.
Neither 1156 nor 1366 are particularly good long term sockets, as it's been heavily hinted that next year they'll be replaced. Intel don't seem to think sockets should last very long. However, your chance of a reasonable upgrade part at the very end of 1366 is not insignificant.
One good reason to go for 1366 (at your budget) is the motherboards, though. Even fairly basic X58 motherboards have a lot going for them. The more basic 1156 motherboards (P55 etc) often seem a bit lacking, and the more expensive ones approach the cost of a 1366 setup anyway. Your budget seems to have quite a bit of room for manoeuvre!
The stock cooler is a bit lame, don't do it. It will just about do its official job of stopping your CPU from turning to expensive red-hot goo, but it makes a lot of noise for what it does, and still gets a bit too hot if you really hammer the CPU. Once it's in it's much more of a pain to switch. I have a similar (i7 920 + 5850) build to the one you plan, and I regret not buying something mightier from the word go.
If you want to combine multimedia use with a fast processor and awesome modern graphics card you may have to think carefully about case cooling if you don't want a lot of distracting airflow noise. It's all too easy to jam in too many fast fans and get a nastily loud computer. Plan on lots of slow fans (and a way to control them) rather than a few fast ones, it's much easier on the ear.
You do not need a 750W PSU for two 5850s. A good 650W one will do.
These are the heatsinks I'd recommend.
* Cooler Master Hyper 212 Plus £20
Probably not the best option for quiet.
* Titan TTC-NK85TZ/V2 Fenrir £30
* Scythe Mugen 2 £39
Would need to buy a retention kit for LGA1156
* Prolimatech Megahalems Rev B £46
Would have to buy fans as well as thermal paste. Probably the one I'd pick.
* Noctua NH-U12P SE2 £54
I suggest you look for a board that has SATA 6Gbps ports, such as the Gigabyte GA-X58A-UD3R, as I feel this is a good upgrade path to have.
At the moment I'm reading a review of the two hex cores that AMD have just released, the platform cost would be lower and you might get better performance, so I will come back when I've read it with anything that I've learnt.